This invention relates to well cementing, and more particularly to the cementing of wells having elevated temperatures.
In the drilling of wells, for example oils wells, wells penetrating sources of geothermal fluids and the like, it is standard practice to utilize a cement to hold the well casing in position and to selectively block or plug portions of the strata through which the well extends so as to prevent the escape of undesirable fluid into the well bore or the loss of drilling muds and the like. When so used, the cement, as an aqueous slurry, is pumped into the annular space between the walls of the bore hole and the casing and permitted to cure so as to form a hardened cement mass which provides the reinforcing and plugging functions.
The cements utilized in drilling operations are formulated so as to be sufficiently slow setting to permit pumping and yet be sufficiently resistant to degradation under the elevated temperature and pressure conditions encountered in the wells. The American Petroleum Institute has promulgated specifications for testing cements to insure that they meet certain minimum requirements with respect to strength, permeability, settling time and the like. These cements are referred to as oil well cements.
To resist the temperatures and pressures normally encountered in wells, the oil well cements have been developed to provide the maximum physical properties under the severe temperature and pressure conditions encountered in relatively deep wells, such as oil wells. These cements are conventionally Portland-type cements to which have been added one or more various additives, such as mica, blast furnace slag, alumina and various special reactive sands, which are designed to improve the mechanical strength and the thermal and chemical resistance of the set and hardened cement. Although satisfactory for conventional well operations, these cements have proven deficient particularly in the case of ultra-deep wells and geothermal wells where temperatures in excess of 400.degree. F. (200.degree. C.) may be encountered. Under such conditions conventional cements quickly increase in porosity and lose compressive strength which may lead to a blowout. Such well blowouts are highly undesirable and can prove to be extremely dangerous, as well as costly to repair.
Accordingly, oil well cements, particularly those used in geothermal wells and in ultra wells, that is wells sunk to depths on the order of 25,000 feet or more, must have the ability to effectively maintain an adequate compressive strength and density and low porosity, even under high temperatures and pressures and in the presence of steam and hot brine. In addition, since the useful life of a typical well is measured in terms of 10 to 30 years, a good oil well cement must operate under the aforementioned severe conditions for a substantial period of time, preferably as long as the life of the well. However, while recent studies have indicated that oil well cements presently in use have effective lives of on the order of 5 to 10 years when utilized in geothermal wells, it was noted that the strength and permeability of the oil well cement were seriously deteriorated in a period of as short as four years thus raising the possibility of a well blowout and increasing the frequency of replacement and maintenance.
My U.S. Pat. Nos. 4,069,870 and 4,114,692 and my copending application Ser. No. 844,642, now U.S. Pat. No. 4,144,077, disclose a method for cementing high temperature wells using a cement system containing up to 20 weight percent of a low volatile material, carbon-containing cement additive, such as anthracite, uncalcined coke, calcined coke, raw oil shale and burned oil shale. Best results are obtained in the disclosed method when the cement system contains between about 6 and about 15 weight percent of these carbon-containing additives. Although the cement systems and method disclosed therein are superior to conventional cement systems, the cost of the carbon-containing additives required may render these cement systems more expensive than conventional cement systems. A need exists for a relatively inexpensive cement system which is suitable for cementing high temperature wells.
Accordingly a primary object of this invention is to provide a method for cementing high temperature wells using a relatively inexpensive cement system.
Another object of this invention is to provide a method for cementing high temperature wells using relatively small amounts of a cement additive, which amount is effective to impart to the cement mass formed therefrom an improved resistance to degradation of its ultimate compressive strength and permeability under the high temperature and pressure conditions in the well.
Yet another object of this invention is to provide a method for cementing high temperature geothermal wells in which the hardened cement mass formed by the method resists degradation when exposed to hot geothermal fluids.
Further objects, advantages and features of this invention will become apparent to those skilled in the art from the following description and claims.